The present invention relates to a magnet roll for use as a delivery means for a magnetic developer in an electrophotographic apparatus, an electrostatic recording apparatus etc., and a method of producing the magnet roll. Particularly, the present invention relates to a magnet roll which does not suffer from breakage or cracking, and a method of producing the magnet roll in which a cylindrical resin-bonded magnet is formed by injection molding integrally with one shaft and the other shaft is press-fitted into a bore of the cylindrical resin-bonded magnet at the opposite end, thereby reducing the number of manufacturing steps and production cost.
In conventional electrophotographic apparatus, electrostatic recording apparatus, etc., a magnet roll used as a developing roll or a cleaning roll generally has a structure as shown in FIG. 4. The magnet roll 1 comprises a cylindrical permanent magnet 2 such as a resin-bonded magnet provided with a plurality of magnetic poles (not shown) on an outer surface thereof, and a shaft 3 coaxially secured into a central bore extending axially through the cylindrical permanent magnet 2.
A pair of flanges 4, 5 are rotatably mounted to the opposite ends of the shaft 3 via bearings 6, 6. A hollow cylindrical sleeve 7 is fixed to the flanges 4, 5 to surround the cylindrical permanent magnet 2. Incidentally, the flanges 4, 5 and the sleeve 7 are made of a non-magnetic material such as aluminum alloys, stainless steel, etc.
In case the magnet roll 1 is employed in the developer roll 8 as shown in FIG. 4, the magnet roll 1 and the sleeve 7 rotate relative to each other. For instance, the magnet roll 1 is kept stationary while the sleeve 7 secured to the flanges 4, 5 are allowed to rotate relative to the magnet roll 1. By this construction, a magnetic developer is attracted onto an outer surface of the rotating sleeve 7 by a magnetic attraction force of the cylindrical permanent magnet 2 and conveyed into a developing region for carrying out the development of an electrostatic latent image on an image-bearing member (not shown) to obtain a developed toner image. Incidentally, the magnetic developer is usually of a one-component type including a magnetic toner, or of a two-component type including a toner and a magnetic carrier.
In the magnet roll 1 described above, the cylindrical permanent magnet 2 is generally made of a sintered magnet such as a ferrite magnet, or a resin-bonded magnet. The resin-bonded magnet is primarily used when the weight reduction of the developer roll 8 is desired. When the cylindrical permanent magnet 2 is made of a resin-bonded magnet, the magnet roll 1 is produced, for instance, by (1) blending a ferromagnetic powder (usually ferrite powder) and a polymer material (usually rubber or plastics) to prepare a mixture, (2) charging the mixture into a cavity of an injection-molding die, in which the shaft 3 as an insert is held in place, to shape a cylindrical resin-bonded magnet 2 fixed to the shaft 3 while applying a magnetic field thereto, (3) cooling and solidifying the cylindrical resin-bonded magnet 2 in the die and then removing an integral body composed of the cylindrical resin-bonded magnet 2 and the shaft 3 from the die, and (4) magnetizing the integral body in the anisotropic direction to form magnetic poles on the cylindrical resin-bonded magnet 2, thereby obtaining the magnet roll 1.
A variety of thermoplastic resin materials have been used to form the cylindrical resin-bonded magnet 2 of the magnet roll 1. However, since the cylindrical resin-bonded magnet 2 is of such an elongated shape that a ratio of a length L to a diameter D.sub.1 is 3 or more, a crystalline resin having a modulus of longitudinal elasticity of 1.times.10.sup.5 kg/cm.sup.2 or higher should be generally utilized to prevent the deformation of the cylindrical resin-bonded magnet 2 and reduce the molding time.
When the electrostatic latent image is developed, it is necessary to apply a bias voltage to the sleeve 7 of the developer roll 8 to prevent fogging on a recording medium or to achieve a reversal development. For this reason, the shaft 3 is generally made of a metal such as steel, stainless steel, etc. and is connected at one end thereof, for instance at an end adjacent to the flange 4, to a voltage source so that a necessary bias voltage is applied via the shaft 3 and the bearings 6, 6 such as sintered bearings, roller bearings, etc. to the sleeve 7.
However, in the magnet roll having such a structure, it is likely that the cylindrical resin-bonded magnet 2 breaks or cracks in its boundary region with the shaft 3 due to a large difference in coefficient of linear expansion therebetween. For instance, the shaft 3 made of steel has a coefficient of linear expansion of 1.1.times.10.sup.-5 cm/cm/.degree. C. and the cylindrical resin-bonded magnet 2 has a coefficient of linear expansion of about 3-4.times.10.sup.-5 cm/cm/.degree. C. (refer to Japanese Patent Publication No. 5-37331). When heated during the injection-molded process and thereafter cooled, the integral body is caused to shrink in the axial direction. In this case, a shrinkage of the cylindrical resin-bonded magnet 2 is larger than that of the shaft 3 so that breakage or cracking is likely to take place in a boundary region between the cylindrical resin-bonded magnet 2 and the shaft 3. Particularly, when the size of the magnet roll 1 is reduced, the thickness of the cylindrical resin-bonded magnet 2 must be reduced correspondingly, for instance, to such an extent that a ratio of an inner diameter D.sub.2 to an outer diameter D.sub.1 is 0.4 or more. As a result, the tendency of breakage or cracking is further accelerated.